Ionic Liquid-Impregnated Metal-Organic Frameworks for CO₂/CH₄ Separation

A large set of distinct ionic liquid (IL)-impregnated metal-organic framework (MOF) composites were produced by a direct-contact method to study their performance as sorbents for gas separation applications. The IL anion/cation impact on the sorption capacity and ideal CO₂/CH₄ selectivity were fully detailed. A reproducible methodology and rigorous characterization were defined to evaluate the IL impact on the IL@ZIF-8 performance. Results show that the IL impregnation was successful, the ZIF-8 structure is conserved after IL incorporation, and the microporous composites are thermally stable at the working temperatures. CO₂ and CH₄ adsorption-desorption equilibria in the composites were measured at the temperature of 303 K and up to 16 bar of pressure. The respective data were then compared with that obtained for pristine ZIF-8. At high pressure, all composites show reversible, although inferior, gas uptake (total pore volume loss due to IL pore occupation/blockage). At low pressure, because of synergistic effects arising from IL-MOF interactions, one composite displays superior CO₂ uptake compared to ZIF-8. Four IL@ZIF-8 composites show distinct low-pressure trends from ZIF-8, due to their IL structure/size, with an increase in the selectivity that can be above 40% at 0.5 bar. An IL-free basis analysis was also assessed considering a normalization of the gas uptake per gram of ZIF-8 in the composites. This shows that ILs do have an impact on the adsorption capacity of the composites. A new approach, based on the materials' pore volume as a key factor, is discussed toward the sorption data of the IL@ZIF-8 composites. Through mapping of the composites data, it is possible to understand the effect of the IL for high- and low-pressure applications. The results obtained herein indicate that IL@MOF composites are potential alternative materials for low-pressure gas separation.